1. Field of the Invention
The present invention relates to a polarization detection device in an integrated optical waveguide device usable in, for example, a magneto-optic information recording and reproduction device, and a method for producing the same.
2. Description of the Related Art
Magneto-optic disks have conventionally been a target of active research and development as a rewritable high-density recording medium. Information is reproduced by detecting the rotation of the polarization direction of light reflected by the magneto-optic recording medium, the polarization of the light being caused by a Kerr effect.
Since the rotation in the polarization direction caused by the Kerr effect is microscopic, realization of a satisfactory signal-to-noise (S/N) ratio requires, for example, a high-precision photodetector or differential detection. As such, bulk-type optical systems including a photodetector, a prism, a mirror and a lens are conventionally used. The bulk-type optical systems have a problem in that positional alignment among the optical elements is difficult. The bulk-type optical systems have another problem in that it is difficult to reduce the size and weight of an apparatus including the system.
Recently, to solve the drawbacks of the bulk-type optical systems, waveguide-type photodetector semiconductor devices have been developed, in which a detecting optical system is integrated on a thin film waveguide. FIG. 17 shows a schematic structure of such a waveguide-type photodetector, i.e., a conventional waveguide-type magneto-optic information recording and reproduction device including a detecting integrated element 95.
The magneto-optic information recording and reproduction device includes a light source 91 including a semiconductor laser or the like; a light converging optical system located so as to converge light from the light source 91 on a magneto-optic information recording medium 92, the light converging optical system including a collimator lens 93 and an object lens 94; a detecting integrated element 95 for detecting light reflected by the magneto-optic information recording medium 92; and a prism coupler 96 provided on the detecting integrated element 95 between the collimator lens 93 and the object lens 94 for causing the light incident on the prism coupler 96 from the collimator lens 93 to be reflected on a bottom surface of the prism coupler 96 so as to propagate toward the object lens 94 and for guiding the reflected light from the magneto-optic information recording medium 92 toward the detecting integrated element 95 as waveguide mode light.
Next, with reference to FIG. 18, the detecting integrated element 95 will be described in detail. The detecting integrated element 95 includes a blank area A and a dotted area B. The area A is a first waveguide area. The prism coupler 96 for guiding the reflected light from the magneto-optic information recording medium 92 is provided on a top surface of the first waveguide area A. The area B is a second waveguide area. The second waveguide area B is provided so as to cause photocoupling together with the first waveguide area A.
Photodetectors 101 and 102 are provided on one end of the area B, and waveguide light converging elements 103 and 104 are provided on another end of the area B opposite to the one end. The waveguide light converging elements 103 and 104 guide the reflected light from the magneto-optic information recording medium 92 to the photodetectors 101 and 102. The photodetectors 101 and 102 are included in a focusing error signal (Fo signal) detection section 105.
The area B further includes a third waveguide area 108. The third waveguide area 108 is a mode splitter for reflecting TE mode light of the reflected light from the magneto-optic information recording medium 92 and refracting TM mode light of the reflected light. The area B also includes photodetectors 106 and 107, respectively, for detecting the TE mode light and the TM mode light. The photodetectors 106 and 107 are included in a magneto-optic signal (MO signal) detection section 109.
The first effective indices of the TEo mode light and TMo mode light in the first waveguide area A are Ne1 and Nm1, respectively, and second effective indices of the TEo mode light and TMo mode light in the second waveguide area B are Ne2 and Nm2, respectively. The first effective indices Ne1 and Nm1 are substantially equal to each other, and the second effective indices Ne2 and Nm2 are different from each other.
Next, the operation of the waveguide-type photodetector shown in FIG. 17 will be described.
Light emitted by the light source 91 is collimated by the collimator lens 93 and is incident on the prism coupler 96. The incident light is reflected by the bottom surface of the prism coupler 96 and is converged on the magneto-optic information recording medium 92 after passing through the object lens 94. The returning light which is reflected by the magneto-optic information recording medium 92 is again incident on the prism coupler 96 after passing through the object lens 94. The reflected light from the magneto-optic information recording medium 92 is coupled to the first waveguide area A via the prism coupler 96 to become waveguide light.
The light guided in the first waveguide area A is optically coupled to the second waveguide area B and then guided to the photodetectors 101 and 102 via the waveguide light converging elements 103 and 104. The light guided in the first waveguide area A and optically coupled to the second waveguide area B is also guided to the photodetectors 106 and 107 via the third waveguide area 108. In other words, the waveguide light is guided to the focusing error signal detection section 105 and the magneto-optic signal detection section 109. Therefore, a focusing error signal is detected by the focusing error signal detection section 105, and a magneto-optic signal is detected by the magneto-optic signal detection section 109.
With reference to FIG. 19, a photocoupler for performing photocoupling will be described. On a waveguide layer 112, a first gap layer 113 having a refractive index lower than that of the waveguide layer 112 is provided. On the first gap layer 113, a second gap layer 115 having a missing end portion is provided. The prism coupler 96 is provided on a portion of the second gap layer 115 and over the first gap layer 113 with an adhesive layer 116 interposed therebetween. The refractive index of the prism coupler 96 is substantially equal to the refractive index of the adhesive layer 116 and is higher than the refractive index of the waveguide layer 113. The second gap layer 115 has a suitable thickness so that the light is coupled to the first gap layer 113 through the missing end portion and also the light, which is once coupled to the first waveguide layer 113, is prevented from coupling to the prism coupler 96 to come out of the prism coupler 96.
In general, a plurality of devices are formed simultaneously on one substrate. In order to divide such a plurality of devices into chips, cleavage of the crystalline substrate can be sometimes utilized. In most cases, the substrate is diced with a diamond blade. However, a large amount of dust is generated in the case of dicing and adhesion of the dust to the surface of the substrate causes a problem of deterioration of performance of the waveguide element.
As a solution to this problem, Japanese Laid-Open Publication No. 4-142503 describes that adhesion of the dust to the substrate is prevented by forming a protection film on a surface of a substrate and removing the protection film after dicing.
With reference to FIG. 20, a mode splitter for splitting the propagation light into TEo mode light and TMo mode light will be described. In order to prevent loss, the thickness of an end portion of a second waveguide layer 122 in the second waveguide area is gradually reduced, and the end portion acts as a first coupling section. The angle of the incident light guided to the end portion with respect to the TEo mode light is larger than a threshold angle. Accordingly, the TEo mode light is reflected and the TMo mode light is refracted.
Hereinafter, with reference to FIGS. 21A and 21B, structures and general production methods of the waveguide-type photodetectors 101, 102, 106 and 107 of FIGS. 18 will be described. FIGS. 21A and 21B are cross-sectional views of each of the waveguide-type photodetectors 101, 102, 106 and 107. FIGS. 21A and 21B show different structures usable for the waveguide-type photodetectors 101, 102, 106 and 107.
First, a structure of the waveguide-type photodetectors 101, 102, 106 and 107 will be described. As shown in FIG. 21A, an N.sup.- epitaxial layer 132 having a P.sup.+ region 132a formed by diffusing boron or the like is provided on an N.sup.+ Si substrate 131. The P.sup.+ region 132a acts as a light receiving section of a light receiving element. On the N.sup.- epitaxial layer 132, a thermal SiO.sub.2 film 133 formed by thermal oxidation is provided. The thickness of the thermal SiO.sub.2 film 133 is reduced on the P.sup.+ region 132a. On the thermal SiO.sub.2 film 133, a waveguide layer 134 formed of a first waveguide layer or a second waveguide layer (or a waveguide layer 134' in which a first waveguide layer and a second waveguide layer are integrated) is provided.
An interconnection 135 is provided so as to be connected to the P.sup.+ region 132a via an opening formed through the thermal SiO.sub.2 film 133 and the waveguide layer 134 (or 134'). A gap layer 136 is provided so as to cover the waveguide layer 134 (or 134') and the interconnection 135. A rear electrode 137 is provided on a rear surface of the N.sup.+ Si substrate 131.
Each of the waveguide-type photodetectors 101, 102, 106 and 107 having the above-described structure is formed in the following manner.
First, the N.sup.- epitaxial layer 132 is grown on the N.sup.+ Si substrate 131, and then the thermal SiO.sub.2 film 133 is formed thereon. A part of the thermal SiO.sub.2 film 133 included in a portion which will act as a photodetecting section is removed by etching or the like, and the resultant laminate is left in a high temperature atmosphere containing boron or the like. Thus, boron is diffused in the N.sup.- epitaxial layer 132 from the removed portion, thereby forming the P.sup.+ region 132a. Next, the waveguide layer 134 (or the waveguide layer 134') is formed on the thermal SiO.sub.2 film 133. The opening is formed through the waveguide layer 134 (or 134') and the thermal SiO.sub.2 film 133 in positional correspondence with the P.sup.+ region 132a.
Then, an electrode material is formed on the waveguide layer 134 (or 134') and patterned, thereby forming the interconnection 135. The gap layer 136 is then formed so as to cover the waveguide layer 134 (or 134') and the interconnection 135. On the rear surface of the N.sup.+ Si substrate 131, the rear electrode 137 is formed. In this manner, each of the waveguide-type photodetectors 101, 102, 106 and 107 is produced.
In order to improve the photodetection efficiency of each of the waveguide-type photodetectors 101, 102, 106 and 107, the gap layer or buffer layer 136 has a tapered structure, in which the thickness of the gap layer 136 is gradually reduced starting from a front side of the waveguide-type photodetector on which the waveguide light is to be incident. Due to such a structure, the waveguide light is received by the waveguide-type photodetector using a waveguide tapering part M. The received light is sent to an external control circuit (not shown) through the interconnection 135, thereby performing photodetection.
In the case where a light receiving element such as a photodiode provided on the N.sup.+ Si substrate 131 is used as the light receiving section of the waveguide-type photodetector, the interconnection is provided in order to send an electric signal such as an electric current corresponding to the amount of the light received by the light receiving element to an external device. The interconnection is usually formed of a metal material on an insulative film on the N.sup.+ Si substrate 131.
Hereinafter, the interconnection 135 and a structure in the vicinity of the interconnection 135 will be described.
In the waveguide-type photodetectors 101, 102, 106 and 107 having the structure shown in FIG. 21A, the interconnection 135 is formed between the waveguide layer 134 (or 134') on the thermal SiO.sub.2 film 133 and the gap layer 136 and is connected to the P.sup.+ region 132a acting as the light receiving section of the light receiving element through the opening formed through the waveguide layer 134 (or 134') and the thermal SiO.sub.2 film 133.
In this case, the gap layer 136 acts as an upper cladding layer for the waveguide layer 134 (134') and also acts as a layer for effectively protecting the interconnection 135 against shortcircuiting, mechanical damages, physical contamination, corrosion and the like. The interconnection 135, which is isolated from the N+Si substrate 131 by the thermal SiO.sub.2 film 133 as well as by the waveguide layer 134 (134'), can maintain the electrostatic capacitance between the interconnection 135 and the N.sup.+ Si substrate 131 at a lower level compared to an electric wire isolated from an N.sup.+ Si substrate only by a thermal SiO.sub.2 film.
In FIG. 21B, each of the waveguide-type photodetectors 101, 102, 106 and 107 includes another gap layer 138 provided between the interconnection 135 and the waveguide layer 134 (134'). The interconnection 135 is provided between the gap layers 138 and 136 and is connected to the P.sup.+ region 132a acting as the light receiving section of the light receiving element through an opening formed through the gap layer 138, the waveguide layer 134 (134') and the thermal SiO.sub.2 film 133. In such a structure, the gap layer 138 further reduces the electrostatic capacitance between the interconnection 135 and the N.sup.+ Si substrate 131.
Recently, waveguide-type photodetectors have been demanded to have a higher-speed response and higher integration for a different use from the use in the integrated element; i.e., for use in a bulk-type optical system. Since the waveguide-type photodetectors are produced in parallel with integrated circuits used in an external control circuit, the structures of the waveguide-type photodetectors have become more complicated. FIG. 22A shows an example of such a waveguide-type photodetector.
The waveguide-type photodetector shown in FIG. 22A includes a thermal SiO.sub.2 film 142 acting as a mask for impurity diffusion and metal layers 143 acting as stoppers for etching and also acting as wires in an integrated circuit section in the vicinity of a light receiving section 141. The waveguide-type photodetector further includes, in the vicinity of the light receiving section 141, an anti-reflection silicon nitride film 144, an interlevel insulative nitride film 145, and protection films 146 for protecting the integrated circuit and the metal layers 143. These elements form a step portion having a total height of several micrometers with respect to the light receiving section 141. In FIG. 22A, a lead portion of the interconnections is omitted.
The protection films 146 and the metal layers 143 can be removed, but the anti-reflection silicon nitride film 144 is indispensable due to the function thereof. The thermal SiO.sub.2 film 142 is also indispensable in order to protect the PN junction. Even in a most simple structure shown in FIG. 22B, a step portion having a height of about 1 .mu.m is unavoidably formed by the thermal SiO.sub.2 film 142.
A step portion of such a height is not a problem for general use, since the light for signal detection is incident from free space.
With reference to FIGS. 23A through 23E, the reason why such a step portion is generated during the production of a waveguide-type photodetector will be described.
As shown in FIG. 23A, a thermal SiO.sub.2 film 152 is formed on a silicon substrate 151. Known as methods for forming the thermal SiO.sub.2 film 152 are dry oxidation by which the silicon substrate 151 is heated in an oxygen flow, and vapor oxidation by which the silicon substrate 151 is heated in an oxygen flow containing vapor. The thermal SiO.sub.2 film 152 formed in such a manner is patterned by a photoresist or the like and etched to form a thermal SiO.sub.2 mask 152a shown in FIG. 23B. Then, impurity diffusion is performed toward the silicon substrate 151 from the etched-away portion, thereby forming an impurity diffusion region 151a shown in FIG. 23C.
In the case where such high temperature processing is performed, a thermal oxide film 153 shown in FIG. 23C is newly formed. In order to produce the waveguide-type photodetector in parallel with the integrated circuit, a SiO.sub.2 film 154 is further formed by CVD or the like as shown in FIG. 23D. Then, the resultant laminate is etched to form an opening 155 through the SiO.sub.2 film 154 and the thermal oxide film 153 as shown in FIG. 23E. An etching pattern for the first etching (to obtain the SiO.sub.2 mask 152a) and an etching pattern for the second etching (to obtain the opening 155) are offset from each other by 2 to 3 .mu.m due to insufficiency in the mask aligning precision and etching precision. Such offset causes a step portion 156 (FIG. 23E).
The photocoupler having the structure shown in FIG. 19 has the following problems. In the case where the adhesive layer 116 is viscous, it is difficult to adjust the amount of the adhesive. In the conventional photocoupler, no good attempt is made for efficiently filling the gap between the prism coupler 96 and the first waveguide layer 113 with an optimum amount of adhesive for the adhesion. There is no specific structure, either, for observing the adhesive which is being put into the gap.
When an excessive amount of adhesive is used, the adhesive overflows and extends onto a side surface of the prism coupler 96 or goes into a holding device (not shown) for the prism coupler 96.
When an insufficient amount of adhesive is used, the adhesive layer 116 contains air bubbles or a space. Since the light is scattered by the air bubbles or the space, the coupling efficiency is lowered.
Moreover, since contraction of the adhesive causes the prism coupler 96 to receive the stress from the adhesive while the prism coupler 96 is being supported by the second gap layer 115, the prism coupler 96 can be undesirably delaminated. Thus, the yield and reliability are lowered.
By the dicing method used for producing a conventional waveguide element, the protection by the protection layer is provided only to the circumferential surface of the waveguide. In the case where the prism adheres before dicing, the surface of the prism is contaminated by the dust generated by dicing, thus decreasing the photocoupling efficiency.
A dielectric layer provided on the substrate on which the integrated circuit is formed for forming a waveguide element has problems in that, when the thickness is not sufficient, the layer below the dielectric layer is exposed in the smoothing process and thus the propagation loss increases due to the contamination and scratches of the surface, resulting in deterioration of the characteristic of the waveguide element.
Moreover, the integrated circuit including a waveguide layer, a buffer layer, a gap layer and the like has serious problems of changing the characteristics of the waveguide element by the film stress and of causing cracks. In order to alleviate the film stress, a layer doped with phosphorus or boron can be provided. However, such a dopant can also undesirably cause a propagation loss.
As described above, a photodetector which copes with the recent demand for high-speed response and high integration has a step portion in the vicinity of the light receiving section thereof. In consideration thereof, production of a so-called waveguide-type photodetector, which includes a waveguide layer provided on the photodetecting section and guides the propagation light to the semiconductor substrate on which the photodetecting section is provided, has problems of difficulty in designing the shape of the buffer layer (dielectric layer) in the vicinity of the step portion and difficulty in precision processing.
With reference to FIG. 24, a practical method for producing the waveguide-type photodetector having the waveguide tapering part M shown in FIGS. 21A and 21B will be described.
In the case where the waveguide-type photodetector has a specific structure, a thermal SiO.sub.2 film 161 is first formed by thermal oxidation, and then a buffer layer 162 is provided on the thermal SiO.sub.2 film 161. A top surface of the buffer layer 162 needs to be smoothed so as to have such a surface roughness that does not influence the waveguide loss. The buffer layer 162 also needs to be processed so as to have such a shape (including thickness, propagation length, tapering angle, and the like) that allows the loss at the step portion to be ignored. When the surface is not smoothed sufficiently, a step portion H' is formed on the waveguide tapering part M. When the thickness of the buffer layer 162 becomes less than a designed value in the vicinity of the step portion H', the light propagating from left to right in FIG. 24 goes out from a waveguide layer 163 toward a semiconductor substrate 164 or free space. This results in the reduction in the coupling efficiency.
A dielectric material used for the buffer layer 162 may increase the surface roughness of the buffer layer 162 when the buffer layer 162 is formed by a certain method. When the surface roughness is excessively large, such a buffer layer cannot be used in a waveguide element. Etching additionally increases the surface roughness, which causes a serious problem.
In a structure including an optical waveguide and a photodetecting section integrated on a semiconductor substrate, the surface of the photodetecting section formed by impurity diffusion is simply covered by a thin film formed of SiO.sub.2 or the like. Accordingly, metal ions entering from outside, especially alkaline ions may undesirably pass through the thin film and reach the diffusion region. In such a case, the charge distribution at the PN junction is adversely influenced, and thus the photodetection function is spoiled.
Only the thin SiO.sub.2 layer exists between the photodetecting section and the waveguide layer. Accordingly, when a glass material containing metal ions, for example, #7059 glass produced by Corning, Inc. is used for the waveguide layer, the metal ions contained in the glass may undesirably pass through the SiO.sub.2 layer and exert an adverse influence similar to the above.
Glass materials can be freely varied in the optical, thermal, mechanical and electric characteristics and processing method by changing the composition there of. However, application of the glass materials to integrated waveguide elements is significantly restricted by the influences of the metal ions contained in the glass materials.
In the waveguide-type photodetector having the above-described structure, the reduction in the coupling efficiency caused at an interface between the photodetecting section and the SiO.sub.2 layer by the light reflection can be a serious problem.
Regarding a polarization splitter, a refraction-reflection type mode splitter shown in FIG. 18 causes a significant difference in the propagating direction of light among different modes. Accordingly, the photodetecting sections provided for the respective modes are positionally discrete. This causes a problem in that different photodetecting sections have different characteristics. In the vicinity of the incident angle which causes total reflection, the reflectance drastically changes. The tolerable range of incident angles is excessively small.
As a solution to these problems in the polarization splitter, a waveguide-type mode splitter is disclosed in Japanese Laid-Open Publication No. 6-82644. The mode splitter disclosed in the above-mentioned publication will be described with reference to FIG. 25.
FIG. 25 is a combination of a plan view and a cross-sectional view of a waveguide-type mode splitter 171. As shown in the part of FIG. 25 which illustrates the cross-sectional structure of the waveguide-type mode splitter 171, the waveguide-type mode splitter 171 includes an optical waveguide layer 173 and a waveguide layer 174 which are provided on a glass substrate 172. The optical waveguide layer 173 is formed of a glass film (#7059, Corning, Inc.) which forms an area B'. The waveguide layer 174 is formed of a TiO.sub.2 film which forms an area A'. The two waveguide areas represented by the areas A' and B' which are included in the waveguide-type mode splitter 171 are coupled to each other by a tapering part C' which is tapering sufficiently slowly with respect to the wavelength of the light.
Since the effective indices of the waveguide areas A' and B' are different from each other, each of different modes (TEo mode light and TMo mode light) incident on the tapering part C' at a certain angle are spatially split at an angle of .theta.' due to the difference between the effective indices of the two areas A' and B'. In the case where the waveguide is formed of, for example, TiO.sub.2 as shown in FIG. 25 or non-alkaline glass, the effective indices of the TEo mode light and TMo mode light having a wavelength of 780 nm are as follows.
Area A' TEo: 1.75; TMo: 1.57 PA1 Area B' TEo: 1.47; TMo: 1.47
When light is incident from the area A' to the area B' in FIG. 25 at an angle of 45.degree. under these conditions, the refractive angles of the TEo mode light and the TMo mode light are respectively 12.3.degree. and 4.0.degree.. The difference is 8.3.degree. (=12.3-4.0).
In the case of an optical pickup, the oscillation wavelength of a semiconductor laser acting as a light source changes in accordance with the ambient temperature. In order to facilitate the assembly and adjustment of the pickup, the tolerance for attaching the parts of the pickup needs to be sufficiently large. For these reasons, a photocoupler which provides sufficient coupling efficiency despite changes in the wavelength or angle of incidence with a small dependency on the polarization is required. In the case of a prism coupler having no special structure, the diameter of the incident light spot is preferably small in general in order to realize such a photocoupler.
However, it is difficult to maintain a beam diameter as small as several micrometers for a long distance since light tends to be expand due to waves thereof. Accordingly, it is unavoidable that the incident light is converged and that a photocoupler is located at the focal point of the converged light.
The converged light diverges when guided to the optical waveguide. Therefore, the above-described waveguide-type mode splitter which utilizes the difference in the refractive index needs to provide a sufficient splitting angle. A substantial mode splitting angle provided by the waveguide-type mode splitter with respect to the diverging light is: (splitting angle with respect to collimated light)-(total diverging angle of incident light).
In the case of an optical pickup having a semiconductor laser as a light source, the light reflected by the disk is converged to the photodetector at the same angle as the radiation angle from the semiconductor laser unless a numerical aperture conversion lens or the like is added. Such an angle exceeds 10.degree.. When the total diverging angle of light in the optical waveguide is set to 10.degree. in consideration of this fact, a mode splitter providing a splitting angle of 8.degree. cannot split the light.
The mode splitting angle is increased by increasing the number of refractions. However, this method is not preferable because one of the modes is partially totally reflected at the interface between the two areas as the size of the mode splitter excessively increases.
For the above-described reasons, the waveguide-type mode splitter disclosed by Japanese Laid-Open Publication No. 6-82644 has a problem that propagation light having a large diverging angle cannot be split.
As described above, a conventional polarization detection device including a photocoupler, an optical waveguide layer for propagating light from the photocoupler, a polarization splitting section for splitting the propagation light into two components by a difference in the refractive index, and a photodetector formed of a semiconductor material for performing photoelectric conversion on the propagation light obtained as a result of the splitting have various problems to be solved as described above.